How is decaffeinated coffee made? The chemistry of coffee decaffeination

Click to enlarge

Caffeine is a stimulant, and the world’s most widely consumed psychoactive drug. Many of us need our morning coffee to be functional, but others prefer to avoid caffeine due to its effects on sleep, restlessness, or pregnancy. Handily, chemists have come up with several ways for us to enjoy coffee without the caffeine. Here, we take a look at the different methods!

The caffeine content of coffee varies by the type of bean. Green robusta coffee beans contain around 2.2% caffeine by mass, whereas the figure for green arabica coffee beans stands at 1.2%. In general, decaffeination doesn’t actually remove all of the caffeine in coffee – just the vast majority of it. In the EU, the maximum caffeine content for decaffeinated coffee is 0.1%, whereas in the U.S. decaffeination must remove 97% of the original caffeine content.

Solvent decaffeination

Solvent decaffeination involves soaking green coffee beans in a solvent to remove the caffeine. There are two types of solvent decaffeination: direct and indirect.

In direct solvent decaffeination, the green coffee beans are steamed to open pores, before soaking in the solvent. Back when solvent decaffeination was first developed, benzene was the solvent used. Today, benzene is recognised as a carcinogen and is no longer used. The two most commonly used solvents today are methylene chloride and ethyl acetate. After soaking in these, the beans are steamed, dried, and roasted. These processes ensure that no residual solvent remains in the beans.

Soaking coffee beans in solvent removes caffeine as it dissolves in the solvent. However, other compounds are also removed from the beans, which can have an impact on flavour. Another type of solvent decaffeination, indirect solvent decaffeination, is an alternative.

Indirect solvent decaffeination involves soaking the beans in hot water to start with. The caffeine and flavour compounds in the beans dissolve in the water. After this, the green beans are removed from the water, which is then mixed with the solvent. The solvent absorbs most of the caffeine, and less of the flavour compounds.

Subsequently, the water and solvent are separated. The beans are then added back into the water to reabsorb some of the flavour compounds they previously lost. As with the direct solvent method, they can then be removed, dried, and roasted.

Carbon dioxide decaffeination

Both methods of solvent decaffeination can lead to some flavour loss in the resultant coffee. One option that improves on solvent decaffeination is decaffeination with carbon dioxide.

You might not instantly think of carbon dioxide as a solvent, as under normal conditions it’s a gas. However, if it’s compressed under high pressure at a little over room temperature, it turns into a supercritical fluid. A supercritical fluid is a substance with the properties midway between a gas an a liquid. If the supercritical carbon dioxide is then mixed with water, it’s very good at dissolving caffeine.

The big advantage of using carbon dioxide for decaffeination is that it’s much more selective for caffeine over other flavour compounds and precursors in coffee beans. The process of extraction involves soaking the coffee beans in water, then loading them into an extractor. Liquid carbon dioxide is then pumped over the bean. Water in a separate chamber removes the caffeine from the carbon dioxide so it can be recirculated, with the process lasting up to 12 hours. As with the previous solvent methods, the beans are then dried and roasted.

Though carbon dioxide decaffeination is superior to other decaffeination methods, a couple of factors limit its use. First, the high-pressure facilities needed require expertise and expense to set up and keep running. Secondly, it’s costly to run – it’s claimed that the extraction method only becomes commerciallyfeasible when 3000 tons of coffee per year are being decaffeinated.

Water decaffeination

Decaffeination with water is another alternative. There are two different methods used: ‘Swiss water decaffeination’ and ‘French water decaffeination’.

Swiss water decaffeination was developed, as the name suggests, in Switzerland. It involves soaking coffee beans in hot water to extract caffeine, with some flavour compounds also being extracted. The extract is passed through a carbon filter, which selectively removes the caffeine.

At this point, the flavour-charged water can be used to remove caffeine from a new batch of green beans. Because it’s already saturated with flavour compounds, mainly caffeine is removed from the new batch of beans, preserving the original flavour.

Alternatively, French water decaffeination can be used. This also involves soaking the beans in hot water, this time for up to 24 hours. The beans are removed and dried, while the water is passed through a carbon filter to selectively remove the caffeine. The caffeine-free, flavour-rich water is then added back to the dry beans, which absorb it. They are then re-dried and roasted.

Other methods

Though these are the main methods used in decaffeination of coffee, they’re not the only options. A number of other methods exist. Some are variations on one of the above themes, such as extraction using nitrogen oxide as a solvent. Others emerging from research are more surprising, such as microorganisms that metabolise caffeine being used for decaffeination.

Please note: none of the graphics on this site are intended for a specific chemistry syllabus, and it should not be assumed that they comprehensively cover any portion of required content for particular qualifications.

The Compound Interest Book

The chemical structures in Compound Interest's graphics are created using PerkinElmer's ChemDraw® Professional v15 desktop software. Includes PerkinElmer copyright material. Reprinted with permission. All rights reserved. ChemDraw® may be purchased online here.